Edge gating is commonly used in the manufacture of slender elongate molded articles such as pipettes or a syringe barrels. The cavity defining the molded article usually has only one gate through which molding material flows in a direction that is generally transverse to the longitudinal axis of the molded article. Pressure within the cavity from molding material flowing through the gate on only one side of the part can adversely affect part filling and/or part geometry. For example, when examining the fill rate of such parts, the side of the part on which the gate is located generally fills first, thus creating an angled flow front as the molding material advances away from the gate. Further, if injection pressure is high enough, the mold core, which defines the inside of the molded article, can be deflected away from the gate. This problem is exacerbated in molded articles that are particularly long, or have a slender core that is not supported at its distal end. The result of such core deflection is molded articles having uneven wall thickness. In molding applications, such as pipette molding and syringe barrel molding, this uneven wall thickness may cause non-uniform part shrinkage which may result in volumetric discrepancies between molded parts as well as an overall increase in the number of defect parts which do not conform to the tolerance requirements of the end user.
One solution to avoiding the aforementioned difficulties with edge gating is to reduce injection pressure and/or increase part fill time; however, this comes at a cost of reduced productivity. Another solution is to inject molding material into each cavity via two mold gates, one on each side of the molded article. Both German patent DE 29902185U and U.S. Pat. No. 7,214,053 disclose edge gating application in which groups of four mold cavities are fed molding material from two sides by groups of four nozzles. The mold cavities are evenly spaced around a first pitch circle and the nozzles are evenly spaced around a second larger pitch circle with the spacing of the nozzles offset by 45 degrees relative to the orientation of the mold cavities. In the resulting arrangement each mold cavity receives molding material from two injection nozzles and each injection nozzle delivers molding material to two mold cavities. While the problem of core shift is reduced in this configuration, the cavitation density of the mold suffers as a result of unused space at the center or the pitch circle. Furthermore, should the cavity/nozzle groupings be increased to, for example, groupings of eight cavities and eight injection nozzles spaced about pitch circles that are used in the above described configuration the cavitation density of the mold is adversely affected as the unused space at the center of each pitch circle increases greatly with an increase in pitch circle diameter.
As such, a need exists in the art for an edge gating apparatus that reduces the above described problems while effectively using the available space inside the mold.